Phased array antenna systems are widely used in radar, electronic warfare and high data-rate communications applications. They are sometimes controlled by networks of optical fibers and other photonic components such as lasers, fiber splitters/combiners, and photodetectors. These control networks mainly utilize delay line networks such as the ones shown in FIG. 1. There are two types of delay line networks which differ in the way time delays are implemented. In the network switched architecture of FIG. 1a, of which the Rotman lens is an example, entire networks of delay lines are switched in/out by a single switch. In the in-line switched architecture of FIG. 1b, there are several delay lines within each fiber as well as a switch to select them. If F is the number of fibers and P the number of delay states, the network switched architecture requires one switch with P states, and the in-line switched architecture F switches with P states. Both require 1×P splitters to access P delay states, and F×1 combiners to vector sum the outputs. For both types of networks, the signal passes through one 1×P splitter, one switch, and one F×1 combiner, so the losses are expected to be comparable.
The number of photodetectors required in the network can be a major cost driver so it is desirable to minimize it. To achieve this, one can place a single photodetector at position A in FIGS. 1a and 1b, after the F×1 combiner which vector sums the fiber signals. However, if all fibers carry the same optical wavelength, as it is the case in most prior art systems, the different signals will interfere and unwanted noise will appear on the detected carrier envelope. In order to avoid this optical coherence problem, photodetectors can be placed at positions B so that photodetection occurs prior to summation, and the optical carriers never interact. However, a large number of photodetectors is then required and cost is greatly increased.
To solve optical coherence problems, while still minimizing the number of photodetectors required, this invention utilizes multiple optical wavelengths. This reduces photodetector count from F×P to 1 in the network switched case (FIG. 1a), and from F to 1, in the in-line switched case (FIG. 1b).
Furthermore, in accordance with this invention, the lossy splitters/combiners that form the actively switched prior art networks of FIGS. 1a and 1b, are replaced by a passive Wavelength Division Multiplexing (WDM) network. The 1×P splitters and F×1 combiners are replaced with WDMs, and the functions performed by active switches are realized by separating wavelength groups with passive WDMs. Optical losses in a 1×N WDM are less than in a 1×N splitter or combiner for N>6, so in most practical cases losses can be substantially reduced.
Prior art photonic networks require active switching, the use of a large number of photodetectors, and inclusion within the network of lossy splitters and combiners. In many cases the prior art also requires specialized or unique optical components.
Prior photonic beamforming art such as described in U.S. Pat. No. 5,861,845 (Wideband Phased Array Antennas and Methods) alludes to using multiple wavelengths to avoid optical coherence effects, but losses are still high in the combiners which vector sum the optical signals. Patent application Ser. No. 09/383,819 (Phased Array Antenna Beamformer) describes a passive receiver network for multiple beams which employs WDMs for beam scanning and delay line selection. However, it does not address optical coherence problems, and uses three-dimensional fiber optics based delay line networks (fiber Rotman lens) which are hard to fabricate. It also utilizes lossy combiners for signal summation.
The present invention addresses and solves these problems in a simple, unified manner, and can be implemented using standard ITU (International Telecommunication Union) components developed commercially for fiber optics data networks, and two-dimensional SOS (Silicon on Sapphire) fabrication techniques.